Adding to the above:


MPH = (RPM X Tire Diameter) / (Final Transmission Gear Ratio X Rear Axle Ratio X 336)

Desired cruising RPM (factors: best engine torque - best gas mileage - empirically to taste)
Tire diameter (in inches)
Final gear ratio of transmission (transmission input to transmission output)


For speedometer:
Change the cable drive gear in the transmission or purchase an external speed adapter.
Calibrating the speedometer gauge itself is best left to a professional gauge restorer.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."